The rapidly increasing device densities in electronics dictate the need for efficient thermal management. If successfully exploited, graphene, which possesses extraordinary thermal properties, can be ...commercially utilized in polymer composites with ultrahigh thermal conductivity (TC). The total potential of graphene to enhance TC, however, is restricted by the large interfacial thermal resistance between the polymer mediated graphene boundaries. We report a facile and scalable dispersion of commercially available graphene nanoplatelets (GnPs) in a polymer matrix, which formed composite with an ultrahigh TC of 12.4 W/m K (vs 0.2 W/m K for neat polymer). This ultrahigh TC was achieved by applying high compression forces during the dispersion that resulted in the closure of gaps between adjacent GnPs with large lateral dimensions and low defect densities. We also found strong evidence for the existence of a thermal percolation threshold. Finally, the addition of electrically insulating boron-nitride nanoparticles to the thermally conductive GnP-polymer composite significantly reduces its electrical conductivity (to avoid short circuit) and synergistically increases the TC. The efficient dispersion of commercially available GnPs in polymer matrix provides the ideal framework for substantial progress toward the large-scale production and commercialization of GnP-based thermally conductive composites.
A comparative study was conducted on composite materials having various nanocarbon fillers of different dimensionalities, namely, 1D carbon nanotubes (CNTs), 2D graphite nanoplates (GNPs), and 3D ...graphite. Comprehensive mechanical, electrical and rheological studies illustrated the complexity of selecting the optimal nanocarbon filler. We found that the mechanical performance of the composite is optimal near the percolation threshold concentration of the filler for all the nanocarbons. The 1D CNTs strongly affected the electrical conductivity and reinforcement of the composite, yielding a narrow range of optimal performance at the lowest filler concentration (0.15 wt%), albeit at the cost of high viscosity. The 2D GNPs demonstrated a wider range of reinforcement with a milder influence on the viscosity at a moderate GNP concentration (3.5 wt%). The 3D graphite filler exhibited similar behavior to that of GNPs, although at a much higher concentration (25 wt%). We introduced a robustness factor as a measure of the filler concentration range at which a valuable reinforcing effect is achieved; this factor increases with the filler dimensionality. These contradicting dimensionality effects are condensed into a figure of merit that takes into account the rheological effect, the mechanical enhancement, and the filler concentration and robustness.
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Reinforcing a fine-grained cement-based matrix with a textile fabric is an emerging approach in civil engineering, termed Textile Reinforced Mortar (TRM). We propose to load the ...ultra-high-performance matrix with nano-additives, i.e., a crystalline admixture (CA) or CA plus alumina nanofibers. The mix was designed to yield flow values - in the fresh state - suitable for retrofitting, and to provide high compressive strength. Examination of the durability of the TRM elements exposed to salty environments unexpectedly showed enhanced tensile strength (<81%). This is owing to the growth of salt crystals at the matrix–textile interface resulting in stronger bond strength, as imaged by scanning electron microscopy, identified by acoustic emission measurements, and validated by pull-off tests.
•The developed textile reinforced mortar composites were durable in a salty environment.•Nano-additives enhanced the mortar's compressive strength and the stress at the first crack of the textile reinforced mortar.•Textile–mortar matrix bond strength was enhanced in samples cured in NaCl solution.•Salt-cured vs. tap-water-cured textile reinforced mortars showed more fine cracks with higher energy.
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Size and morphology distributions are critical to the performance of nano-drug systems, as they determine drug pharmacokinetics and biodistribution. Therefore, comprehensive and ...reliable analyses of these properties are required by both the US Food and Drug Administration (FDA) and European Medicines Agency (EMA). In this study, we compare two most commonly used approaches for assessing the size distribution and morphology of liposomal nano-drug systems, namely, dynamic light scattering (DLS) and cryogenic-transmission electron microscopy (cryo-TEM); an automated quantitative analysis method was developed for the latter method. We demonstrate the advantages and disadvantages of each of these two approaches for a commercial formulation of the anti-cancer drug doxorubicin - Doxil®, in which the drug is encapsulated, mostly in the form of nano-rod crystals. With increasing drug concentration, these nano-rods change the shape of the liposomes from spherical, before drug loading, to prolate (oval), post drug loading. Cryo-TEM analysis provides a detailed size distribution of both the liposomes (minor and major axes) and the nano-rod drug. Both these values are relevant to the drug performance. In this study, we show that at elevated drug concentration (2.75 mg/ml) the drug grows mainly along the major axis and that this high concentration can result, in some cases, in liposome rupture. We show that the combination of cryo-TEM and DLS constitutes a reliable tool for demonstrating the stability of the formulation in human plasma at body temperature, a characteristic that is crucial for achieving therapeutic efficacy.
Dispersion and exfoliation of carbon nanotubes (CNT) by water soluble dispersants such as surfactant, polymer or protein is a key step toward the application of carbon nanotubes in composite ...materials, biochemical and biomedical applications. Upon dispersion, the solution phase separates into dispersed nanotubes in the supernatant and a precipitate phase including carbonaceous impurities but also nanotubes and dispersants. Yet, simple but accurate tools for measuring the concentrations of the constituents are not available. In most studies a comparison between CNT suspensions is based on ocular observation or on UV−visible measurement of a featureless spectrum at single wavelength. Such measurements are complex since both nanotubes and most dispersants absorb along the whole UV−visible spectrum and an overlap of their signals occurs. In this paper we employ chemometric techniques to evaluate the pH effect on the concentration of both dispersant (protein−bovine serum albumin, BSA) and single-wall nanotube (SWNT) from a full UV−visible spectrum of aqueous solutions. We find strong correlation between the conformation of the protein and its dispersion efficiency.
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Surfactants have been widely employed to debundle, disperse and stabilize carbon nanotubes in aqueous solvents. Yet, a thorough understanding of the dispersing mechanisms at molecular ...level is still warranted. Herein, we investigated the influence of the molecular structure of gemini surfactants on the dispersibility of multiwalled carbon nanotubes (MWNTs). We used dicationic n-s-n gemini surfactants, varying n and s, the number of alkyl tail and alkyl spacer carbons, respectively; for comparisons, single-tailed surfactant homologues were also studied. Detailed curves of dispersed MWNT concentration vs. surfactant concentration were obtained through a stringently controlled experimental procedure, allowing for molecular insight. The gemini are found to be much more efficient dispersants than their single-tailed homologues, i.e. lower surfactant concentration is needed to attain the maximum dispersed MWNT concentration. In general, the spacer length has a comparatively higher influence on the dispersing efficiency than the tail length. Further, scanning electron microscopy imaging shows a sizeable degree of MWNT debundling by the gemini surfactants in the obtained dispersions. Our observations also point to an adsorption process that does not entail the formation of micelle-like aggregates on the nanotube surface, but rather coverage by individual molecules, among which the ones that seem to be able to adapt best to the nanotube surface provide the highest efficiency. These studies are relevant for the rational design and choice of optimal dispersants for carbon nanomaterials and other similarly water-insoluble materials.
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•High-Precision dispersibility curves determined for 1D (MWNT) and 2D (GnP) nanocarbons.•Anionic surfactant SC is a more effective and efficient dispersant than the nonionic ...TX-100.•SEM shows a good debundling/exfoliation degree of the nanocarbon in all dispersions.•Normalized individual curves by dispersed nanocarbon concentration unveil a master curve.•Results underpin common (kinetic-based) mechanisms for dispersing MWNTs and GnPs.
Most applications of nanocarbons, such as carbon nanotubes and graphene, require that they are well-separated and well-dispersed in a liquid phase. Intensive efforts have been put on exfoliating and dispersing nanocarbons in aqueous solvents, typically using amphiphilic dispersants and sonication/centrifugation procedures, alongside a drive to fundamentally understand and rationally optimize these processes. Herein, we employed a robust method to separate and disperse multiwalled carbon nanotubes (MWNTs), and graphene nanoplatelets (GnPs) either from bulk graphite or from pre-formed GnP powders, using rigorously controlled processing conditions. An ionic (sodium cholate) and a nonionic (Triton X-100) surfactant were used as dispersants. Our aim was to determine high-precision dispersibility curves (concentration of dispersed nanomaterial versus initial surfactant concentration) for the different nanocarbon/dispersant systems, characterize morphologically the dispersed particles and compare the mechanisms of exfoliation of 1D and 2D nanocarbons at molecular level. Typically bell-shaped dispersibility curves with a plateau were obtained, and from the latter several quantitative metrics were extracted that permitted reliable comparisons between nanocarbon/surfactant systems. Scanning electron and atomic force microscopies allowed to characterize the suspended particles in the as-obtained dispersions, namely the MWNT bundle width and GnP dimensions (mean lateral size and layer number). Under fixed conditions (in particular, delivered energy per carbon mass), MWNTs are dispersed in much higher yields, by two orders of magnitude, than GnPs. However, and significantly, a master curve for the dispersibility was obtained, implying that common fundamental features underpin the dispersing process, irrespective of nanocarbon (1D or 2D) or surfactant (ionic or nonionic) types.
Extensive work has been invested in the design of bio-inspired peptide emulsifiers. Yet, none of the formulated surfactants were based on the utilization of the robust conformation and self-assembly ...tendencies presented by the hydrophobins, which exhibited highest surface activity among all known proteins. Here we show that a minimalist design scheme could be employed to fabricate rigid helical peptides to mimic the rigid conformation and the helical amphipathic organization. These designer building blocks, containing natural non-coded α-aminoisobutyric acid (Aib), form superhelical assemblies as confirmed by crystallography and microscopy. The peptide sequence is amenable to structural modularity and provides the highest stable emulsions reported so far for peptide and protein emulsifiers. Moreover, we establish the ability of short peptides to perform the dual functions of emulsifiers and thickeners, a feature that typically requires synergistic effects of surfactants and polysaccharides. This work provides a different paradigm for the molecular engineering of bioemulsifiers.
Sonication-assisted graphene production from graphite is a popular lab-scale approach in which ultrasound energy breaks down graphite sheets into graphene flakes in aqueous medium. Dispersants ...(surfactant molecules) are incorporated into the solution to prevent individual graphene flakes from reaggregating. However, in solution these dispersants self-assemble into various structures, which can interfere with the characterization of the graphene produced. In this study, we characterized graphene dispersions stabilized by a family of pyrene-based surfactants that facilitate a high exfoliation yield. These surfactants self-assembled to form flakes and ribbonsshapes very similar to those of graphene structures. The dispersant structures were present both in the graphene dispersion and in the precipitate after the solvent had been evaporated and could therefore have been mistakenly identified as graphene by electron microscopy techniques and other characterization techniques, such as Raman and X-ray photoelectron spectroscopy. Contrary to previous reports, we showedby removing the dispersants by filtration and washingthat the surfactants did not affect the shape of the graphene prepared by sonication.
The substantial heat generation in highly dense electronic devices requires the use of materials tailored to facilitate efficient thermal management. The design of such materials may be based on the ...loading of thermally conductive fillers into the polymer matrix applied - as a thermal interface material - on the interface between two surfaces to reduce contact resistance. On the one hand, these additives enhance the thermal conductivity of the composite, but on the other hand, they increase the viscosity of the composite and hence impair its workability. This in turn could negatively affect the device-matrix interface. To address this problem, we suggest a tunable composite material comprising a combination of two different carbon-based fillers, graphene nanoplatelets (GNPs) and graphite. By adjusting the GNP:graphite concentration ratio and the total concentration of the fillers, we were able to fine tune the thermal conductivity and the workability of the hybrid polymer composite. To facilitate the optimal design of materials for thermal management, we constructed a 'concentration-thermal conductivity-viscosity phase diagram'. This hybrid approach thus offers solutions for thermal management applications, providing both finely tuned composite thermal properties and workability. We demonstrate the utility of this approach by fabricating a thermal interface material with tunable workability and testing it in a model electronic device.
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